Chicago — Illinois's primary – along with those of 21 other states – is this Tuesday. But Lou Smith and Sam Sirko cast their votes for Sen. Barack Obama a week ago, during a three-week period in which Illinois offers early in-person voting.

While Mr. Sirko says his choice was made some time ago, Mr.. Smith only decided recently. "I was more teetering," says Smith, a physical therapist. The New Hampshire and South Carolina campaigns were what "really turned me off to Clinton."

In a primary season in which momentum and timing are coveted, it can be hard for candidates to know when to target certain voters. Thirty-four states now offer some form of no-excuse early or absentee voting, including about half of the Super Tuesday states. In delegate-rich California, half of all primary votes may be cast by mail the month before Feb. 5.

With early voting a relatively recent phenomenon, it's unclear just how it will affect voters. Undecided voters will often wait until Election Day to make their decision, but even stalwart supporters may make an early choice that would have changed had they waited.

In Florida's primary, about a quarter of all ballots were cast early, which seemed to benefit Sen. Hillary Rodham Clinton. She won the state with 50 percent of the vote, compared with Senator Obama's 33 percent. But while absentee balloting favored Senator Clinton, exit polls showed a more even matchup between the two candidates among last-minute deciders.

Key Super Tuesday states allowing some form of no-excuse early voting include California, Illinois, Arizona, Georgia, New Jersey, New Mexico, and Utah. (In states with caucuses, early voting won't be a factor even if allowed.)

In California, some 2.3 million voters have already voted by mail, says Stephen Weir, president of the California Association of Clerks and Election Officials. Based on the turnout he anticipates, he predicts that just under half of all ballots will be cast early and believes that a quarter of the vote is already in. Mr. Weir says several candidates bought lists of those permanent absentee voters, and have been working to lock in early votes.

In general, people who vote early tend to be more decided, slightly older, and more partisan, says Robert Stein, a political science professor at Rice University. Because of that, he notes, some candidates will change their message in the weeks leading up to the actual primary.

"You basically divide the electorate into two groups, early and election, and you work early voters because you know they're more partisan, and you work election-day voters, because they're less partisan and less likely to vote," Dr. Stein says.

While voters tend to say early voting is more convenient, some critics say it depresses voter turnout, increases the chance for fraud in the case of voting by mail, and means voters may not have all the information they need. Some Californians, for instance, may have already cast votes for Rudolph Giuliani or John Edwards, both of whom dropped out of the race this week.

"What if in the 2004 election, on the Saturday before the election, Osama bin Laden had been captured ... or the stock market tanked to the tune of 1,000 points?" asks Curtis Gans, director of the Center for the Study of the American Electorate in Washington. "Many things could change [people's] minds."